Metal-air electrochemical power sources, particularly Al-air batteries and fuel cells with alkaline electrolyte are suitable for electric vehicles, unmanned aerial vehicles (UAV), reserve and emergency power supply and other applications.
The electrochemical reaction in Al-air system yields products which are transferred through the electrolyte. Aluminum—air system with alkaline electrolyte has a great electrochemical capacity (about 8 k Wh/kg). As a result, this system requires large amount of the electrolyte compared to other metal-air systems. The use of the large amount of the electrolyte in the reaction is achieved by its circulation through the electrochemical cell. However such Al-Air systems have few disadvantages. One of them is formation of aluminum hydroxide gel in the electrolyte as a product of current generation during electrochemical reaction. This process increases the electrolyte density and decreases its ionic conductivity which results in a low electrolyte utilization coefficient (about 0.3 Ah/ml) and correspondingly to low energy density value.
One of the ways to overcome this problem is by filtration or separation of the formed aluminum hydroxide gel from the “pure” electrolyte [D. Linden, T. B. Reddy, Handbook of Batteries, third edition, p.p. 38.43 and U.S. Pat. No. 4,994,332]. Another way to overcome the formation of aluminum hydroxide gel is by adding to a circulating alkaline electrolyte after few hours of discharge (when electrolyte is coming to saturation by aluminum hydroxide) a seed material such as aluminum oxide or aluminum salts, which could be a driving force for the precipitation out of the already formed Al(OH)3 [U.S. Pat. No. 5,260,144]. The seed material is added to the electrolyte by a time responsive automatic seed injector after certain battery discharge time. U.S. Pat. No. 4,663,249 suggests to control the aluminum hydroxide gel concentration by contacting the electrolyte with acids or salts what finally results in formation of insoluble electrolyte compounds, which could be later separated or isolated from the electrolyte.
The above techniques trying to reduce the amount of aluminum hydroxide gel do not solve electrolyte deterioration problems and noticeably increase the need for methods to avoid, reduce or remove aluminum hydroxide precipitate from the alkaline electrolyte.